1. Field of Invention
The present invention is concerned with implant materials for medical use that have a highly biocompatible surface layer of titanium or titanium alloys. More specifically, the implant materials have high biocompatibility or bioactivity due to appropriate electrochemical treatments on their surfaces.
2. Prior Art
The implant materials having titanium or titanium alloys as their surface layer exhibit so little biocompatibility or bioactivity that they are confronted with a great problem of displacement or loosening unless they are provided with those properties.
One of the attempts against such problem is to develop a group of implants hat are coated with a layer of calcium phosphate comics such as hydroxy apatite due to plasma spraying or coated with an apatite layer due to dipping the materials in a multicomponent solution containing calcium ions and phosphate ions as the major ingredients, followed by calcining. However, some shortcomings are found in those surface modification treatments: The former method of plasma spraying needs temperatures higher than 1000xc2x0 C. and a much more expensive apparatus as well as a more complicated procedure; the latter method of dip-coating not only requires a longer period of treatments but attains insufficient adhesion between the implant metal surface and the as-deposited coating. As a result, it exhibits only inferior fixation bonds to living tissues.
Thus a method of providing direct fixation between the metal surface and living :tissues has been developed where titanium or titanium alloy substrates or those having those metals as the surface layer are soaked in a highly concentrated alkaline solution to yield a hydrated gel layer, which layer can deposit apatite when it is placed under body environment after calcination at a higher temperatures. An example is described in Japanese Patent Application Laid-Open No. Hei 1-275766. This method, however, embodies some drawbacks in the procedure like danger of using a highly concentrated alkaline solution as well as unexpected distortion of shape and damage in mechanical properties due to heating up to as high as 600xc2x0 C. Such treatment raises the cost of the implants.
Another method of promoting strong fixation of the titanium or titanium alloy implants has been proposed where the implants are soaked in a warm aqueous solution of hydrogen peroxide containing some metal ions to yield a hydrated titania gel layer that involves those metal ions hence exhibits an improved ability of depositing apatite when soaked in a body fluid or simulated body fluid with calcium and phosphate ions as the major ingredients. The present inventors studied the processes and disclosed such a method of highly efficient and improved treatment in Japanese Patent No. 2795824.
Accordingly, the object of the present invention is to provide the surface of metallic substrates with high bioactivity by virtue of a more simple method and treatment at temperatures lower than the method of soaking the in such aqueous hydrogen peroxide solutions described above. The present inventors have further studied so that they could provide metallic materials having mechanical strength enough for implants, excellent bonding to living tissues, and superior fixation.
The present invention then is to provide the titanium and titanium alloy substrate with bioactivity due to electrochemical treatments as the solution for the problems in the prior art. That is, either titanium or titanium alloys or materials having those as their surface layer are employed as the cathode and electrolyzed in calcium ion-containing aqueous solutions, through which treatment the substrates are covered with a layer with calcium ion absorbed or involved.
Another invention is concerned either with such titanium or titanium alloys or materials with those as their surface layer that are first electrochemically treated in a calcium ion-containing aqueous solution employing those as the anode to yield an oxide hydrated layer on titanium or titanium alloys (the layer is to be simply denoted as a hydrated oxide layer, and sometimes is present in the form of a hydrated gel layer) before those metallic materials are subject to cathodic polarization (cathodic treatment) so that the cathodic treatment ensures absorption or involvement of calcium ions in the hydrated oxide layer. Such consecutive anodic and cathodic treatments hence yield biomedical implant materials exhibiting both excellent bioactivity and outstanding compatibility to living tissues by virtue of the hydrated oxide gel layer having superior ability of spontaneously depositing apatite layer under body environment.
The medical implant materials concerned with the present invention, then, hold calcium ions inside or on the hydrated oxide layer due to changing the polarity of the electrode made of the implant materials from anode to cathode. The state of the calcium ions is held either being absorbed or being involved. Appropriate cathodic potential applied to the cathode made of the titanium or titanium alloy implants may be rarer(more negative) than xe2x88x920.5 V vs. the Ag/AgCl reference electrode. The potential is hereafter expressed by the value in volts vs. Ag/AgCl electrode unless otherwise described. Favorable effects are obtained for the potentials xe2x88x921.5 to xe2x88x923 V. The potential rarer than or exceeding xe2x88x923 V leads to rigorous electrolysis of water and violent evolution of oxygen or hydrogen gas at each electrode hence is inadequate. Appropriate anodic potentials may be larger than +0.5 V. Although a higher anodic potential gives rise to a thicker hydrated oxide layer the potentials of +1.5 V or greater are most favorable for the sake of stronger adhesion among the implant, the hydrated oxide layer, and the apatite layer deposited under body environment.
The implants suitable for the treatment in the present invention can be a wide range of natural or artificial organic polymers and metallic materials, and are not restricted in any means. Most favorably chosen polymer materials are such single component natural or artificial polymers or multicomponent composites consisting of a few of them that are mechanically strong and tough enough to be employed as implant materials for medical use. The polymers of special preference include polyethylene, polypropylene, polytetrafluoroethylene (Teflon(copyright), Goretex(copyright)), polyvinylchloride, and polycarbonate, to name a few. For attaining bioactivity those organic polymer materials should be provided with a surface layer of titanium or titanium alloys due to some surface modification procedure. Several methods can be employed for the surface modification such as plating, spraying, ion injection, PVD, CVD, ion mixing, and other common methods.
The metallic substrates suitable for the treatment of the present invention are not only titanium or titanium alloys but cobalt-chromium alloys, stainless steel, and others that have a surface layer of titanium or titanium alloys. Naturally, the implants made of titanium or titanium alloys need no such prior surface modification. The examples suitable for the biomedical implants include a series of metallic materials of titanium systems, that is, titanium and titanium alloys. The ingredient metals for the titanium alloys can be selected among such common metals as Al, Sn, Fe, Co, Ni, Cr, Cu, V, Mo, W, Ta, Ag, Zr, or other ones with considering mechanical properties, ease of shaping, or some other factors at manufacturing. Among the alloys, an alloy Tixe2x80x946Alxe2x80x944V is appropriate. The biomedical implant materials should exhibit excellent biocompatibility and bioactivity by virtue of the hydrated surface oxide layer or hydrated gel layer that can spontaneously deposit apatite due to the calcium ions absorbed or involved inside.
Thus, the biomedical implant materials described in the present invention denote a group of solid materials for medical use consisting of the substances mentioned above, and no restriction is applied to shapes or usage as far as they ensures the properties necessary for the use in the human body, no harmfulness, or no toxicity. For example, plates, rods, blocks, sheets, fibers, pellets, or any other shapes will be suitable.
One of the most favorable solutions of calcium ions for the surface treatment of the substrates (implants) is an aqueous solution of calcium nitrate (Ca(NO3)2) with a concentration of 0.001 mol/L or more, and more concentrated one is appropriate for shorter periods of treatment. The less concentrated calcium nitrate solutions than that mentioned above can be effective as, well whereas a longer period of treatment is required. Room temperature is appropriate enough for the temperature of the solution for the electrochemical treatment, and temperatures above 0xc2x0 C. will yield adequate results. Any solutions dissolved with calcium compounds other than calcium nitrate such as calcium chloride or calcium acetate can be employed for the electrochemical treatment.
The cathodic electrochemical treatment applied to the implant materials that are made of titanium or titanium alloys as well as those coated with a titanium or titanium alloy surface layer and have a hydrated oxide layer or a hydrated gel layer derived by a prior treatment with, for example, hydrogen peroxide solutions can also result in the same effects that are expected when it is applied to the implant materials treated with the anodic polarization: the calcium ions can be absorbed on or be involved in the hydrated oxide layer or a hydrated gel layer mentioned above.
Unless they are treated with the electrochemical polarization concerned with the present invention, neither of common titanium or titanium alloys nor any materials having titanium or titanium alloys as the surface layer have such an active hydrated titania gel, Therefore, instead of the hydrogen peroxide solution treatment, the anodic electrochemical treatment is to be so applied to those implant materials that the calcium ions can be more easily held on or in the surface hydrated titania gel layer, is, prior to the cathodic electrochemical surface treatment, the anodic electrochemical surface treatment is applied to the implant materials as they are kept in the same electrochemical cell so that such hydrated oxide layer or hydrated oxide gel layer can be yielded. With such consecutive and simple electrochemical treatments in the present invention, common titanium or titanium alloys or any materials having titanium or titanium alloys as the surface layer can hold calcium ions on or in the surface gel, hence the treatments can provide the materials with bioactivity superior to that derived from only a cathodic electrochemical treatment. Moreover, the present treatments alone result in much more excellent effects without help of a prior treatment with the hydrogen peroxide solutions.
Thus obtained biomedical implant materials covered with the hydrated oxide layer or hydrated oxide gel layer involving calcium ions exhibit prominent ability of spontaneously depositing apatite under body environment. The experiments with a simulated body fluid of the Kokubo recipe (SBF) have indicated that apatite is deposited within only 6 hours after soaking in SBF. Such excellent ability of in vitro depositing apatite definitely assures the implant materials of the superb in vivo biocompatibility and bioactivity, which has not been provided until the biocompatible biomedical implant materials are developed due to the electrochemical treatments in the present invention described above.